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But there's no way to know whether the H5N1 strain of bird flu ever will undergo the series of genetic changes required before it could begin spreading from person to person.

"It is entirely conceivable that this virus is inherently programmed that it will never be able to go efficiently from human to human," Fauci said. "Hopefully the epidemic (in birds) will burn itself out, which epidemics do, before the virus evolves the capability of being more efficient in going from human to human."

Even if it does make the human leap - "big if," Fauci said - that isn't likely to happen overnight. Scientists are closely monitoring the virus for such changes, and an early warning could be if doctors or nurses caring for someone who caught H5N1 from a bird somewhere in the world in turn got sick.

The scientific uncertainty makes for public confusion that has only intensified as the spring migratory season arrives, bringing predictions that an infected bird eventually will reach U.S. shores.

But even if a migrating bird were to drop dead of H5N1 in Long Island Sound, the virus isn't likely to make inroads here that it has elsewhere in the world, Fauci stressed.

Consider: H5N1 is known to have infected about 190 people, and killed about half of them, mostly in developing countries. Virtually all caught it from close contact with sick chickens, often small children or local farmers who handled not just the birds but their droppings, which contain high amounts of virus. They probably didn't wash their hands before touching faces or preparing food, Fauci explained.

Here, few people have such intimate contact with chickens. Also, industrialized nations have more protected poultry flocks that lessen the chances of a migrating bird infecting chickens or turkeys - and commercial farmers are paid for the poultry that they must quickly kill to wipe out any H5N1 infection.

"It won't be what you see in countries in which there is no regulation, in which there is no incentive to compensate farmers, in which the people, who are so poor, when they see their chickens are getting infected they immediately sell them or they don't tell anybody because they don't want them culled. That is a critical issue that is fundamentally different than what we see in Western Europe and that we will see in the United States," Fauci said.

Bird flu already has spread to birds in so many countries that eradicating it would be difficult. But usually, as viruses adapt to spread more easily, they become significantly less virulent, he noted.